Telephone conference circuit



Apl18,1939. J A.PARROTT TELEPHONE CONFERENCE C IRCUIT Filed Nov. 2o,1937 wmv l .r BS@ Tv M LAEQ

Patented Apr. 18, 1 939 TELEPHONE CONFERENCE CIRCUIT John AngellParrott, BMlyll, N. Y., asltnor to American Telephone and TelegraphCompany. a corporation of New York Application November 20, 1937, SerialNo. 175,105

10 Claims. (CL 179-42) This invention relates to telephones systems.More particularly, this invention relates to circuits for holdingconferences between a plurality of parties. Still more particularly,this invention relates to private branch exchange telephone systems,otherwise known as P. B. X systems, and it relates to arrangements forincreasing the number of parties which may be engaged in simultaneousconversation over such systems.

One of the objects of this invention is to increase the number ofparties at a P. B. X system which can participate in a conversation orcon ference. Y i

Another of the objects of this invention is to increase the number ofparties to a telephone conference all of which are served by the P. B. Xsystem and at the same time include at least one party to the conferencewho is not served from the P. B. X system. Y v

This invention will be better understood from the detailed descriptionhereinafter following when read in connection with the accompanyingdrawing showing one embodiment of the invention merely for the purposeor illustration.

In the drawing the reterencecharacters I. 2. 3,

4 and 5 represent a hybrid coilnetwork and the reference characters 6,1, 8, 9 and I0 represent a similar hybrid coil network, both of thenetworks being of well known type and construction. The coil 5 of one ofthe networks is connected to the terminals common to the equal coils Band 'l and 8 and 9 of the other hybrid coil network through a one-wayamplifier II. The coil I0 of the second hybrid coil network is connectedto the terminals common to the equal coils I and 2 and 3 and i of thefirst-mentioned hybrid coil network through an oppositely directedone-way amplier I2. These amplifiers II and I2 may be of any well known'construction.

The terminals at the right of coils 2 and l are connected to a step-downtransformer I3, the coil of fewer turns of which is connected to aplurality of jacks I 4, I5, etc. of any well known type. Only two jacksI4 and I5 are shown at this point for illustration, but it will be clearthat four or more jacks may be paralleled to the two just refered to.The terminals at the left ofcoils I and 3 are similarly connected to astep-down transformer 23, the coil of fewer turns of this transformerbeing connected to a plurality of jacks, two of which are shown merelyfor the purpose of illustration; although it will be obvious that fouror more jacks may be added to the circuit and these would ordinarily beparalleled to jacks 24 and 25 in setting up the circuit oi' thisinvention.

The jacks il, I5, 2l. 2l, etc. are bridged by fixed networks i6, I1, Zi.21. respectively, and these extend to a common bus. Each of thesenetworks comprises an arrangement ofelenients of inductance, capacitanceand resistance interconnected in such a way as to simulate theirnpedance of a telephone subscriber's average extension set andcircuit. The various subscribers telephone sets are designated II, 52.63. 54 etc. and they may be connected by plugs Il, l2, 4,3, M, etc.,respectively, to the jacks Il, I5, 2|, 25. etc.

As any jack, such as M, is connected by an ap propriete cord circuit toan extensionv set, such as Si, the corresponding network. in this casenetwork I6, will be disconnected from the circuit of transformer I3 uponthe insertion of the plug 4I of the ,extension'set circuit into the jackIl. In other words. either a subscribers set or a network simulating itsaverage impedance will always be connected to the system at any one ofthe various positions. Such an arrangement has for its object themaintenance of substahtially constant impedance relationships at alltimes. The impedance conditions during varied operations will bemaintained substantially constant whether the maximum number oftelephone ex? tensions are being used or not.

The terminals at the right oi' coils 1 and 9 extend to a jack l0. Thisjackis employed, as in ordinary P. B. X systems. for connecting thecircuits shown in the drawing to a common circuit 29 which leads to acentral ofllce or other point.

, At the central oilice the modiiled P. B. X system maybe switchedeithermanually or automatically from one subscriber-'S line to anothersubscriber's line in any manner well known in the art. The trunk linecircuit 29 extending `from the central ofiice to jack lil may also beused for the transmission of ringing and other super7 visory signals inany manner known to the art and this feature forms no part ofthisinvention.

The terminals at the left of coils C and '8 may be connected by means ofa switch `II or permanently wired to either oi` two circuits. One ofthese circuits includes a network 3,2 simulating the impedance of 'theline 29 extending between the jack 30 and the central office and stillan other network 33-34 which is intended to simulate the circuit orcircuits at the central ofllce to which the arrangement shown in thedrawing may be connected. The network 32 may include. for example.series resistive elements and shunt capacitive elements. The networkIS-pl may comprise series resistive and capacitive eleF ments as shown,convenient values of which may be 600 ohms and 2 mf., respectively. Thelatter values are given merely by way of example and are therefore notto be considered as the only values suitable for these elements nor arethey to be treated as limiting the invention in any way.

The other circuit to which the terminal at the left of windings 6 and 4-may be connected may include a cable pair 3l similar to the pair 29connected to jack 30. The cable pair 35 may be terminated at the centraloffice in a network 36--31 similar to the network 33-34. Either oi' thetwo circuits extending to switch 3| will balance the hybrid coilarrangement 3--1-4--3--Iil against the circuit extending between jack 30and the central office.

The circuit described hereinabove differs from the ordinary P. B. Xcircuit in several respects: (l) It includes amplifiers II and I2.whichare not found in the ordinary P. B. X system.

(2) One of the pairs of windings 'of the hybrid coil arrangement at theleft, as for example, windings 2 and 4, are connected to a transformeri3 and to the telephone extension sets or equivalent networks connectedto the transformer I3 instead of to an ordinary balancing network. (3)The other of the pairs of windings of the hybrid coil arrangement at theleft is connected to a plurality of parallel extension y telephones orequivalent networks through a step-down transformer 23 and not to but asingle telephone extension. These are some of the departures of thearrangement of this invention from the prior art P. B. X system.

Telephone currents coming in from the central ofllce over conductors 29pass over the winding I0- to the amplifier I2. Here they` are amplifledand are then transmitted through the windings I. 2, 3 and 4. At thesewindings the currents divide approximately equally. half of the currentsflowing through transformer I3 to the various extension telephonesconnected to jacks I4, I5, etc., the remaining currents flowing throughtransformer 23 to the telephone extension sets connected to jacks 24,25, etc.

Outgoing telephone currents pass over winding 5, through amplifier Il tothe points common to windings 6 and 1 and 8 and I and then over jack 30and over the cable pair 29 extending to the central onice. 'Ihesecurrents are then transmitted to the subscrlbers set to which thecentral ofce circuit is switched.

The arrangement described will readily permit twelve (or more) P. B. Xextension telephones to be connected simultaneously in parallelarrangement to a subscribers telephone set (or sets) reached over atrunk connection from jack 30. These circuits are then suitable for atelephone conference between all of these various parties. The grade oftransmission for the twelve (or more) parties with the circuit describedwill be commercially satisfactory, but the transmission can be improvedwith better balance between the two sides 'of each hybrid coll network.This arrangement represents a substantial improvement over formercircuits which provided satisfactory communication between only twostations and permitted transmission though of poorer grade when a thirdparty was added.

The amplifiers II and I2 are inserted into the circuit for the purposeof compensating for the loss introduced by the numerous bridgedextension telephones. With the arrangement illustrated and described, itwill be observed that the low side of each of the transformers I3 andvdescribed hereinabove.

23 is connected to a group of the telephone extension circuits. Thetransformers I3 and 23 act to reduce and practically eliminatereflection losses between the amplifiers and the buses to which theextension stations are connected.

The various extension telephone sets 5I, 52, 53. 54. etc.. and the linesconnecting these sets to the buses would ordinarily be of the samedesign and manufacture. However, if these sets differ appreciably fromeach other or ii' lines vary considerably in length or in otherproperties. these deviations may be compensated by appropriate changesin the elements of the networks I8, il, 26, 21, etc.. and by suitabletraffic operating practices to assure connection of each type extensionto the proper jack.

In the arrangement illustrated in the drawing, it is desirable thatequal numbers of extension telephone sets, or their equivalent networks.be connected to the two bus-bars. Obviously, the latter may beaccomplished by other means than For example, instead of the jacks andassociated networks shown in the drawing, the bus connectedtotransformer 23 might be bridged by jacks so arranged that when pluggedinto, contacts would close. which connect an appropriate network to thebus associated with transformer I3. This might be employed for all ofthe jacks or for only those jacks to be used for special extensioncircuits which, for example, are appreciably longer than the average orhave different transmission characteristics.

The hybrid coil transformers are illustrated as of the three-windingtype. It will be clear that such transformers are not indispensable incarrying out the principles of this invention. Obviously any otherhybrid arrangement having equivalent properties may be inserted into thesystem in piace of the three-winding transformers shown.

It will be understood that the two one-way amplifiers illustrated may ofcourse be replaced by any two-way amplifier well known in the art. Ifdesired, the amplifier may be of a type such that when amplification oftelephonic or other currents flowing in one direction is taking place,the flow of currents in the opposite direction will be suppressed orprevented.

While this invention has been shown and described in certain particulararrangements merely for the purpose of illustration, it will be obviousthat the invention may be applied to other and widely variedorganizations without departing from the spirit of the invention and thescope of the appended claims.

What is claimed is:

l. The combination of two groups of parallelconnected telephone sets,first and second three winding transformers, two of the windings of therst transformer interconnecting the two groups of sets, a. one-wayamplifier connecting the third winding of the transformer to themidpoints of two of the windings of the second transformer, a secondoppositely directed oneway amplifier connecting the third winding of thesecond transformer to the midpoints of said two windings of the firsttransformer, a trunk circuitlconnected to said two windings of thesecond transformer, and a network connected to segrtwo windings of saidsecond transformer to b nce the trunk circuit.

2.1The combination of two groups of telephf le extension sets, the setsof each group beingv connected in parallel relationship, a hybrid coilnetwork formed of three mutually coupled windings, two transformers eachhaving one of its windings connected to a common pair of the windings ofthe hybrid coil network, the

5 remaining winding of each transformer being connected to one of thegroups of extension sets. a remote telephone set, said remote telephoneset being connected to the midpoints of the common pair of windings ofthe hybrid coil network for transmitting telephone currentssimultaneously to all of the extension sets, said remote telephone setbeing connected to the third winding of the hybrid coil network forreceiving telephone currents simultaneously from all of said extensionsets.

' 3. A system for providing a conference circuit between a plurality oftelephone extension circuits with one or more remote telephone circuits,comprising a plurality of networks, each $0 simulating a telephoneextension circuit, means for paralleling all of said extension circuitsor their corresponding simulating networks into two groups. two hybridcoil transformers, the two groups of extension circuits being connectedto one of said hybrid coil transformers to balance each other, a trunkcircuit to which any one of the remote telephone circuits may beconnected, a network simulating the trunk circuit. said latter networkand said trunk circuit ben ing connected to the other of said hybridcoil tr nsformers to balance each other, and ampli ying means couplingthe two hybrid coil transformers.

4. The combination of two transformers, two

u one-way oppositely directed amplifiers interconnecting saidtransformers, two groups of parallel connected telephone circuitsconnected to one of the transformers so that the groups of telephonecircuits will balance each other, and a 40 telephone line connected tothe other transformer, thereby permitting simultaneous twowaycommunication with good quality over said telephone line and over all ofthe telephone circuits.

45 5. A telephone conference circuit comprising two groups of parallelconnected telephone circuits, a telephone line, means for coupling saidline to all of said telephone circuits, said means including twotransformers, one of said trans- 59 formers interconnecting the twogroups of teleby the addition of a plurality of networks eachcorresponding to one of the telephone circuits, and means for connectingeach of said networks to the parallel circuit with which it isassociated upon the disconnection therefrom of the 10 correspondingtelephone circuits.

7. In a telephone conference system, the combination of twosubstantially equal groups of telephone subscribers' circuits, atransformer coupling said two groups of circuits, a trunk 15 circuit,and means including a two-way amplifier for coupling the trunk circuitwith the transformer.

8. A telephone conference system having, inv combination, a plurality ofgroups of two-way 2o telephone subscribers' circuits, means including atransformer for coupling all of said circuits to each other, a maincircuit over which televphone currents may flow in opposite directions,

and means including an amplifier for coupling 25 said main circuit withsaid transformer coupling means, telephone currents flowing over themain circuit being amplified by said amplifier before being received bysaid subscribers circuits, telephone currents ilowing in saidsubscribers circuits being amplified by said amplifierl before beingtransmitted over said main circuit.

9. The combination set forth inclaim 8 including, in addition, aplurality of networks each simulating one of said subscribers circuits,and means responsive to the removal of .any of said subscribers circuitsfor substituting the corre-

